Account for existing but empty cpus file

If a CPU file exists but is empty, previously we reported this file with a CPU count of 1, which resulted in a single-threaded run.

lib/util.js

@@ -316,7 +316,10 @@ function getCgroupCpuCountFromCpus(cpusFile, logger) {
     }
     let cpuCount = 0;
     // Comma-separated numbers and ranges, for eg. 0-1,3
-    const cpusString = fs.readFileSync(cpusFile, "utf-8");
+    const cpusString = fs.readFileSync(cpusFile, "utf-8").trim();
+    if (cpusString.length === 0) {
+        return undefined;
+    }
     for (const token of cpusString.split(",")) {
         if (!token.includes("-")) {
             // Not a range